I wrote to Castrol about the use of synthetics in higher mileage trucks. My 98 F150 has 105k on it and uses no oil, I want to use synthetic. I wrote to Castrol and here is their response:

Castrol SYNTEC may be used in older, high mileage vehicles. Castrol SYNTEC has been recommended for all U.S. and foreign cars and has passed Industry tests for seal and gasket compatibility. However, while there is no seal compatibility concern in older vehicles, deterioration of the seals, resulting in an oil consumption problem, can arise. Synthetic oil can be more fluid and if there has been deterioration of the seals, a leak may
become more apparent.

Susan Howell
Reference: 110556

 

Good info! Thanks.

 

Last I heard, Castrol Syntec isn't a true synthetic oil. It is a highly refined conventional oil with enough synthetic additives to allow them to legally call it synthetic. It's probably a fine oil, but it's not synthetic.